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Tuesday, June 19, 2018

Astrophysicists try to falsify multiverse, find they can’t.

Ben Carson, trying tomake sense of the multiverse.

The idea that we live in a multiverse – an infinite collection of universes from which ours is merely one – is interesting but unscientific. It postulates the existence of entities that are unnecessary to describe what we observe. All those other universes are inaccessible to experiment. Science, therefore, cannot say anything about their existence, neither whether they do exist nor whether they don’t exist.

The EAGLE collaboration now knows this too. They recently published results of a computer simulation that details how the formation of galaxies is affected when one changes the value of the cosmological constant, the constant which quantifies how fast the expansion of the universe accelerates. The idea is that, if you believe in the multiverse, then each simulation shows a different universe. And once you know which universes give rise to galaxies, you can calculate how likely we are to be in a universe that contains galaxies and also has the cosmological constant that we observe.

We already knew before the new EAGLE paper that not all values of the cosmological constant are compatible with our existence. If the cosmological constant is too large, the universe either collapses quickly after formation (if the constant is negative) and galaxies are never formed, or it expands so quickly that structures are torn apart before galaxies can form (if the constant is positive).

New is that by using computer simulations, the EAGLE collaboration is able to quantify and also illustrate just how the structure formation differs with the cosmological constant.

The quick summary of their results is that if you turn up the cosmological constant and keep all other physics the same, then making galaxies becomes difficult once the cosmological constant exceeds about 100 times the measured value. The authors haven’t looked at negative values of the cosmological constant because (so they write) that would be difficult to include in their code.

The below image from their simulation shows an example for the gas density. On the left you see a galaxy prototype in a universe with zero cosmological constant. On the right the cosmological constant is 30 times the measured value. In the right image structures are smaller because the gas halos have difficulties growing in a rapidly expanding universe.

This, however, is just turning knobs on computer code, so what does this have to do with the multiverse? Nothing really. But it’s fun to see how the authors are trying really hard to make sense of the multiverse business.

A particular headache for multiverse arguments, for example, is that if you want to speak about the probability of an observer finding themselves in a particular part of the multiverse, you have to specify what counts as observer. The EAGLE collaboration explains:

“We might wonder whether any complex life form counts as an observer (an ant?), or whether we need to see evidence of communication (a dolphin?), or active observation of the universe at large (an astronomer?). Our model does not contain anything as detailed as ants, dolphins or astronomers, so we are unable to make such a fine distinction anyway.”

But even after settling the question whether dolphins merit observer-status, a multiverse per se doesn’t allow you to calculate the probability for finding this or that universe. For this you need additional information: a probability distribution or “measure” on the multiverse. And this is where the real problem begins. If the probability of finding yourself in a universe like ours is small you may think that disfavors the multiverse hypothesis. But it doesn’t: It merely disfavors the probability distribution, not the multiverse itself.

The EAGLE collaboration elaborates on the conundrum:

“What would it mean to apply two different measures to this model, to derive two different predictions? How could all the physical facts be the same, and yet the predictions of the model be different in the two cases? What is the measure about, if not the universe? Is it just our own subjective opinion? In that case, you can save yourself all the bother of calculating probabilities by having an opinion about your multiverse model directly.”

Indeed. You can even save yourself the bother of having a multiverse to begin with because it doesn’t explain any observation that a single universe wouldn’t also explain.

The authors eventually find that some probability distributions make our universe more, others less probable. Not that you need a computer cluster for that insight. Still, I guess we should applaud the EAGLE people for trying. In their paper, they conclude: “A specific multiverse model must justify its measure on its own terms, since the freedom to choose a measure is simply the freedom to choose predictions ad hoc.”

But of course a model can never justify itself. The only way to justify a physical model is that it fits observation. And if you make ad hoc choices to fit observations you may as well just chose the cosmological constant to be what we observe and be done with it.

In summary, the paper finds that the multiverse hypothesis isn’t falsifiable. If you paid any attention to the multiverse debate, that’s hardly surprising, but it is interesting to see astrophysicists attempting to squeeze some science out of it.

I think the EAGLE study makes a useful contribution to the literature. Multiverse proponents have so far argued that what they do is science because some versions of the multiverse are testable in our universe, for example by searching for entanglement between universes, or for evidence that our universe has collided with another one in the past.

It is correct that some multiverse types are testable, but to the extent that they have been tested, they have been ruled out. This, of course, has not ruled out the multiverse per se, because there are still infinitely many types of multiverses left. For those, the only thing you can do is make probabilistic arguments. The EAGLE paper now highlights that these can’t be falsified either.

I hope that showcasing the practical problem, as the EAGLE paper does, will help clarify the unscientific basis of the multiverse hypothesis.

Let me be clear that the multiverse is a fringe idea in a small part of the physics community. Compared to the troubled scientific methodologies in some parts of particle physics and cosmology, multiverse madness is a minor pest. No, the major problem with the multiverse is its popularity outside of physics.
Physicists from Brian Greene to Leonard Susskind to Andrei Linde have publicly spoken about the multiverse as if it was best scientific practice. And that well-known physicists pass the multiverse off as science isn’t merely annoying, it actively damages the reputation of science.
A prominent example for the damage that can result comes from the 2015 Republican Presidential Candidate Ben Carson.

Carson is a retired neurosurgeon who doesn’t know much physics, but what he knows he seems to have learned from multiverse enthusiasts. On September 22, 2015, Carson gave a speech at a Baptist school in Ohio, informing his audience that “science is not always correct,” and then went on to justify his science skepticism by making fun of the multiverse:

“And then they go to the probability theory, and they say “but if there’s enough big bangs over a long enough period of time, one of them will be the perfect big bang and everything will be perfectly organized.””

In an earlier speech he cheerfully added: “I mean, you want to talk about fairy tales? This is amazing.”

Now, Carson has misunderstood much of elementary thermodynamics and cosmology, and I have no idea why he thinks he’s even qualified to give speeches about physics. But really this isn’t the point. I don’t expect neurosurgeons to be experts in the foundations of physics and I hope Carson’s audience doesn’t expect that either. Point is, he shows what happens when scientists mix fact with fiction: Non-experts throw out both together.

In his speech, Carson goes on: “I then say to them, look, I’m not going to criticize you. You have a lot more faith than I have… I give you credit for that. But I’m not going to denigrate you because of your faith and you shouldn’t denigrate me for mine.”

And I’m with him on that. No one should be denigrated for what they believe in. If you want to believe in the existence of infinitely many universes with infinitely many copies of yourself, that’s all fine with me. But please don’t pass it off as science.If you want to know more about the conflation between faith and knowledge in theoretical physics, read my book “Lost in Math: How Beauty Leads Physics Astray.”

95 comments:

As a non-physicist, I've always wondered to what extent this kind of argument (that the value of the cosmological constant is finely tuned and that it deserves an explanation) assumes that our current theories are already final theories of everything (and as far as I know, we know they're not). Is this fine-tuning something that is more or less independent of our theories, something that is here to stay in future theories (as could be, say, Bell's inequalities violation, which does not rely on particular theories) or couldn't a future fundamental theory explain it away by reinterpreting the data? I'd like to have a physicist's point of view on this...

The whole point here is that the universe is comoutable in the sense that everything can be computed by a Turing machine. However, I am not really sure this is true. In fact I think that most things are hypercomputable. So I think this simulation may not be able to reveal the whole truth...

Eternal inflation, if I understand it correctly, has a universe constantly in the inflationary state with bubbles of "normal" space-time condensing randomly. If we live in such a bubble, what happens at the boundary? Is there a wave of reheating spreading out into the still inflationary part (eternal Big-Bang)? Is there a wall with a "Caution: Space doubling every 10^-34 seconds beyond this point. Do not touch" sign on it?

I personally equate discussions of the multiverse with arguments about what material epicycles are made out of. Both cases of confusing a model with reality.

"Carson has misunderstood much of elementary thermodynamics and cosmology, and I have no idea why he thinks he’s even qualified to give speeches about physics." Perhaps the author can therefore tell us what qualifies her, as a non physicist who criticizes physicists, to adjudicate physics?

The sentiment of your last paragraph expresses what I have felt for many years. My first subscription to Scientific American had been in 1972. I no longer subscribe because "science writers" who promote science are not the researchers doing science. Researchers used to write the articles themselves. Now, it is a subclass of researchers who seem to be unscientific.

My original science interest had been biology, and, whatever may be true concerning evolution, evolution strongly indicates that mathematics may not be treated as "revealed knowledge".

I received a degree in mathematics because my part-time acquisition of credits arose from my need to work. I could get no acommodation for flexible access to laboratory time from the biology and chemistry departments in my school. But, mathematics ought to have provided an excellent background for a science interest. Unfortunately, I ran into a problem called "the continuum hypothesis". What I have discovered is that there is more mathematics taught in philosophy departments than in mathematics departments.

The conflation of mathematics with the pursuit of a logical calculus is a serious matter since the abstract nature of mathematical discourse means that one must explain words with words in endless circles. So, when one treats mathematics as "revealed knowledge" under the banner of "science", one is no longer constrained by the empirical ground of sensory experience which is what one is trying to explain.

You will find no logician acknowledging that the sixteen truth tables underlying propositional semantics can be organized into the finite affine geometry of similar order. Look up Curtis' Miracle Octad Generator to see how quickly you run into the Mathieu groups of interest to quantum gravity subject matter. Along similar lines compare the order relations for the free Boolean algebra on two generators to the order relations associated with the tetrahedral simplex as described in combinatorial topology. But, when you toss out geometry in favor of words explaining words you break any relation between mathematics and empirical sensory experience.

I see the problems you consider at a much larger scale. Standardized tests select for those who will probably achieve "academic success". That does not mean that the have a scientific attitude toward knowledge. Our schools produce experts who can write papers. This is "useful to someone. But, is it useful for the progress of science?

The results of Barnes et al are a big snore. It really does not tell us anything much different than what can be calculated on a single piece of paper.

So at the risk of being maybe a bit too technical. I will show how to think about this with the back of an envelope. A dark matter halo extends out about 10^5 light years and is about 3 times the mass of the stars in that galaxy. From this I can get a density for the dark matter halo. I can estimate a Ricci scalar curvature from this mass as R = 8πGρ/3c^3 ~ 5×10^{-49}m^{-2}. The cosmological constant is Λ = 1.1×10^{-52}m^{-2}. We then see that locally the presence of dark matter has a curvature that is about 4000 times the curvature of cosmological constant. Using Gauss's law one can show the occurrence of dark matter has a harmonic oscillator potential on matter within the halo and the cosmological constant has a similar term. So the Newtonian potential energy on stars in a galaxy is

V = constant×(Λ/3 - R)×r.

That is negative or binding since the local Ricci curvature is larger than the cosmological constant by a factor of 1200. This is getting remarkably close to Barnes et al's number of 300. Clearly if the cosmological constant were larger there would be no local gravitational clumping. If the cosmological constant were a significant fraction of the DM induced Ricci term it would result in a larger perturbation. One can then use more realistic density functions and do some elementary Newtonian analysis to capture some of the results of Barnes.

I did this with 30 minutes of time, a pen and paper and without a major research grant. Now Barnes et al did some fancy computer work, which is largely what their work is about. However, the results are not that different.

This has on top of these computations has little bearing on the multiverse, even if that concept may be problematic in other ways. Barnes is a major exponent of fine tuning arguments in favor of what amounts to intelligent design. The cosmological constant of the observable universe, here keeping the door open for there being a multiverse, is very small. I keep the door open because without empirical data I really do not know and the multiverse argument is then grist for mill so to speak. Barnes shows there are two orders magnitude variability that is allowed to give galaxy formation. The argument on the probability of observers seems fairly heuristic. Barnes is trying to make a path in favor of a sort of ID argument to say the cosmological constant of the observable universe is somehow unique. Their results do not really have that implication.

As for Ben Carson, the current HUD Secretary, has made various bizarre statements. He refutes the whole big bang cosmology by saying explosions destroy things instead of creating things. He is a religious fundamentalist who has said the Egyptian pyramids were built under the orders of Joseph as granaries. He is an odd man, for as a neurosurgeon he is clearly no dummy, but he strikes me in ways as being some form of idiot savant. He appears to have a penetrating brilliance in that one area and a complete knucklehead with respect to much else.

The Pennsylvania farmer-turned preacher William Miller predicted the Second Coming of Jesus in 1843. When it didn't happen, he said he'd made a slight computational error, and predicted it would take place in 1844 instead. When that also failed, many of his estimated 50,000 to 500,000 followers left the fold, but enough stayed on thinking Miller was somehow correct anyway. That remnant became the Seventh-Day Adventist Church, which today has some 15 million adherents. Ben Carson is one of them.

Dr. Carson has no business even being mentioned on this site, much less showing up at universities to expound on physics. He's a prime example of how religion mixed with politics and science is ruining the world.

Observer a philosophical concept and at least requires a capacity for abstract thought. Higher animals have minds, perceive, and "observe" their environments within the context of their lives, but they are only metaphorically "observers" in scientific or philosophical contexts

I find studies like this fascinating. It says more about the sophistication of computer code than about the multiverse. I commend the authors for their ability to concoct a story in mathematics and realize it in code.

One can hope that some day such efforts will reward us with an insight into physics that will equate to a 21st century equivalent of Einstein's "Happiest Thought". It is clear that, for the moment, some areas physics are suffering from a lack of new directions for research. For what it is worth, my belief is that a critical reexamination of available data and foundation assumptions will lead us out of the current doldrums.

Why would universes collide in the multiverse? Sounds like a dumb multiverse. The multiverse crap is where physicists have jumped the shark like the Fonz in their attempt to totally explain away how a universe like ours can exist without a God to create it. I'm not writing this to inject religion. But some physicists have now gone so far afield in their atheistic zeal that they have created a more unreasonable notion of our existence than God kicked off the Big Bang! Yes, I'm saying it is more reasonable to believe that God blew a puff of breath into the void and out sprang our universe such as it is than to believe in the ridiculously untestable notion of universes we can never see or touch. By the way, I think God is untestable also, just not ridiculous. But thanks EAGLE for discovering what the rest of us with a brain already knew, that the multiverse idea is nuts. There are real unanswered problems in physics that need working on, like quantum fucking gravity for instance!

Physicists from Brian Greene to Lisa Randall to Leonard Susskind to Andrei Linde to Louis Carrol to Stephen Hawkins, Neil Turok to .... have written numerous best selling books about the multiverse mania. I tried to sold all this fairy tale books on eBay but no one want them anymore ;-(

"No one should be denigrated for what they believe in.". people who believe in something that is obviously nonsense and/or potentially harmful should definitely be denigrated. this includes people who promulgate ideas that do harm to theoretical physics as a scientific discipline and lower the respect that the layperson has for the field. if theoretical physics is regarded as being no more valid than a belief in a personal god who can arbitrarily change or ignore the laws of nature, it is a very bad thing for everyone.

"Observer [is] a philosophical concept and at least requires a capacity for abstract thought. Higher animals have minds, perceive, and "observe" their environments within the context of their lives, but they are only metaphorically "observers" in scientific or philosophical contexts".

Then “observer” is the wrong word; poor word choices lead to confusion.

For non-scientists (like me), isn’t the allure of multiverse theories in part that it seems to “naturally” answer the question of “how did the Big Bang happen”?

Describing the “Big Bang” as a one-off event is a hard-sell. Making it just one of many such events is much more persuasive; especially for those who are not religiously-inclined. Multiverses are not the only way to do that, but they seem to be the fashionable option currently.

For those who are religious (Carson et al.) a multiverse is doubly-offensive. Not only does it dispose of any need for their deity, the idea that there could be vast numbers of other universes just like ours (populated with people just like all of us) disposes of any notion of our “specialness”; specialness is an important part of most religious belief these days.

As for myself, I find these ideas interesting, but I’m not convinced.

sean s.

ps: I’m about half-way through “Lost in Math” and enjoying it immensely.

Is multiverse really a fringe idea in the physics community? There's at least three different multiverse theories that crop up: quantum mechanics (everett interpretation), string theory and cosmic inflation theory. And it's a fairly popular topic in science articles written for the public. Doesn't seem that small to me.

"Why would universes collide in the multiverse? Sounds like a dumb multiverse."

There are several different multiverse concepts, as Dr. Hossenfelder has explained here previously and in her book. I think the one referred to is the Eternal Inflation multiverse, in which there are hypothesized to be multiple bubbles in the same universe, each bubble considered a separate universe, and those bubbles could be nearby and could collide.

Each multiverse has been considered logically as an outgrowth of existing, somewhat-confirmed theories (theories there is some empirical evidence for), so they are not dumb - at least, not as dumb as criticizing other people's thoughts before understanding them. Their problem is that no evidence has been found for them. For example, if there are bubbles in eternal inflation, our bubble did not collide with any other bubble in a so-far measurable way. The God hypothesis has some opposite problems; it does not arise from any existing, confirmed theories of physics, and does not explain anything (if there has to be a god to create things, who created the god--it just moves the problem back one step, leaving the why and how questions unaddressed). It shares the problem that when we look scientifically for evidence of it, none is found.

The universe is a very complex thing and who knows what might or might not be true? I am for religious tolerance, but only up to the point where it conflicts with scientific evidence. E.g., those who say the world is flat, or climate change is not real, or Trump is a good President.

During the Apollo Moon project, one of my grandmothers told me, "If God wanted us on the Moon, he would have put us there." That is the attitude I associate with most religions, unfortunately.

In "Lost in Math" Dr. Hossenfelder interviewed some prominent physicists for their thoughts on string theory, super-symmetry, and multiverses. Some said that they had or used to have hopes that some of these things would turn out to be true, because then they would understand the universe better (I think), but I don't recall them expressing an absolute belief such as religions demand (i.e., belief even in the face of contradictions), or claiming there was good empirical evidence for them. We figure things out by trial and error, just as biological evolution developed us, which is to say, there are bound to be errors.

What I'm really saying has almost nothing to do with religion or a personal god. I'm saying this multiverse idea is beyond ridiculous, even MORE so than a belief in a personal god and it is time to get back to real hard physics that is testable.

I watched a pop science show the other day about time. The parallel I see to your article is we seem to have a large number of physicists who grew up on, and loved science fiction. As physicists they appear determined to prove the fantastical things they watched and read as children are now scientifically possible. This is a poor and dangerous way to research and develop scientific theories. About the only thing I’m convinced of is that mathematics can be constructed to predict observations accurately that also contains untestable predictions and interpretations of fantastical, and impossible realities.

No matter how good the model, physicist’s need to seriously consider that just because math says it may be so, isn’t a license to accept unprovable, untestable and bizarre predictions as being scientifically reasonable or responsible. It’s much more likely the math construction is flawed and not entirely representative of reality even when providing a plethora of useful and accurate predictions.

Actually, if Ben Carson is talking about anthropic selection of fundamental constants, he appears to understand the idea perfectly: one big bang after another, until one where everything is tuned just right, so as to allow biochemistry.

Hi Sabine, do you think with all the recent developments in fundamental physics that it's still possible that Einstein's dream of a final theory that determines all the properties of our universe, is still possible.Thanks for all your posts and your book.

If, let's say for argument, some new theory predicts both observable things in places we can observe (better than existing theory, let's say) and non-observable things elsewhere, is that not scientific?

I assume such a theory doesn't exist. I certainly don't have expertise to say! But it seems possible to me that a multiverse or things even more extravagant could come as baggage with new theory that is (despite -- not because of -- the baggage) good theory.

Hi Dr. H. Have not read Suskind on this matter. I was trying to say that being "an observer" in any meaningful scientific context requires more than merely sophisticated sensory apparatus and an integrating consciousness which the higher animals all have. "Observer status" also implies the capacity to abstract and express general principles that describe contents of consciousness independent of of particular instances

Respectfully, I at least partially disagree with you Sabine. As you note, some multiverse possibilities are testable and, based on scientific arguments, they have been falsified. I think it is science to then strive to determine whether a wider range of models - ultimately all of them - are falsifiable. Assuming from the outset that there is no possibility to apply a scientific method is itself unscientific. Moreover, in doing so one may discover new approaches that provide opportunities that at the outset were not available. In principle, one might eventually be able to apply a 'global' (poor choice of word for this discussion!) argument to rule out all multiverse theories (*). Hence, I don't think a requirement that the science has the capability to validate any particular theory is ultimately necessary.

* Yes, you can then expand the multiverse concept, but again, one could potentially eliminate it.

All that said...

- possibly the incompleteness theorem scuppers my argument.

- I think a large fraction of multiverse work is, for the reasons you state, unscientific...

"The idea that we live in a multiverse – an infinite collection of universes from which ours is merely one – is interesting but unscientific. It postulates the existence of entities that are unnecessary to describe what we observe. All those other universes are inaccessible to experiment. Science, therefore, cannot say anything about their existence, neither whether they do exist nor whether they don’t exist."

This is a common straw-man argument. By the same token, we can't believe what GR tells us about the interior of black holes, because we can never get any information from there. The multiverse isn't a hypothesis, it is a consequence of theories which have proved useful in other ways. Not every prediction of a theory has to be testable in order to have confidence in it.

"If the probability of finding yourself in a universe like ours is small you may think that disfavors the multiverse hypothesis. But it doesn’t: It merely disfavors the probability distribution, not the multiverse itself."

This seems to be a misunderstanding on your part. What you say is technically true. Yes, the probability distribution (which, I agree, we don't know) might be really strange, just happening to favour our existence. But, in a Bayesian sense, other arguments make this unlikely.

I could write a book trying to explain this, but since Lewis and Barnes have already done so, please read that and say, exactly, what they get wrong.

It is clear that you don't like the multiverse. But if people point you to counter-arguments, you should at least take them in and try to refute them, not just ignore them because your mind has already been made up.

"I hope that showcasing the practical problem, as the EAGLE paper does, will help clarify the unscientific basis of the multiverse hypothesis."

This is like saying that scientists can't (yet) explain how some feature evolved, so a) evolution is bunk, Darwin was evil, hence Jesus was divine. You can't rule out a general argument on the basis of a failure of one single aspect (even if it is a failure).

"Let me be clear that the multiverse is a fringe idea in a small part of the physics community."

Any idea in physics is a fringe idea in the sense that most people work in some other field. Among scientists who actually work in this field, the multiverse is mainstream. Please feel free to call Lord Rees, the Astronomer Royal, a crackpot.

"A prominent example for the damage that can result comes from the 2015 Republican Presidential Candidate Ben Carson."

Now you are really clutching at straws. Find an idea you don't like and, as evidence against it, find some crackpot who also doesn't like it. The enemy of my enemy is my friend. Logically bunk. I can find some crackpot who believes that all your work is nonsense. What does that prove?

"If, let's say for argument, some new theory predicts both observable things in places we can observe (better than existing theory, let's say) and non-observable things elsewhere, is that not scientific?"

Of course it is. Sabine is attacking a straw man here.

"I assume such a theory doesn't exist."

Sure it does. General Relativity, for example, says what goes on inside black holes.

"But it seems possible to me that a multiverse or things even more extravagant could come as baggage with new theory that is (despite -- not because of -- the baggage) good theory."

Indeed. You have it right. Unfortunately, Sabine has it wrong here. I say "unfortunately" because not only is she usually right, but she doesn't care what other people think and usually looks at things in detail before expounding on them. But with regard to the multiverse, fine-tuning, and related issues, she not only misses the boat, but ignores what serious scientists have done in this area.

Before the Big Bang theory became accepted, some physicists believed that the universe had been here for an infinite amount of time. Now, I suppose, that idea has morphed into the notion that we live in a multiverse which is the product of an infinite number of big bangs -- and the whole thing has been around for an infinite amount of time.But years ago I remember reading an argument against such a past infinity: if the past was infinite, we never would have gotten here, because you can never get to the end of an infinity. If we got to the end of it, it wasn't an infinity. Any thoughts about that?

For all your talk about supposed straw men, your complaint that I use Carson to show a crackpot "agrees" is a big fat straw man. I have used him to showcase what's the damage to science, not to say that what he argues is correct.

Regarding your comment about black holes, entirely different case because we do have access to the initial data. Besides, as you certainly know people do question whether what GR tells us about the inside of black holes is correct.

I find it hard to shake of the feeling that the multiverse is science fiction, along with Dyson spheres and jumping into hyperspace. I was shocked when I realised just how many of my friends took it as scientific fact and then I realised that they were simply excited about the idea. Which is why a lot of science write on this topic - its just easy sensationalist copy. Maybe we just need better science writers.

(Please delete if this has been posted twice. I'm just unsure if this has been published. It would be nice of Wordpress to confirm this when clicking on publish).

I don't like double posting; I feel I am over extending a welcome. There are a few comments I can think of here with regards to various comments by others.

The multiverse is not a dumb idea. It has not been falsified, but of course it has not been demonstrated either. It is an extension of inflationary cosmology that has some low sigma, about σ ~ 3, support from CMB data, and back in 2015 it was thought for a while that maybe it was clinched with BICEPII. It was realized that dust in this galaxy might polarize EM radiation in a way to emulate the B-modes inflation generated gravitational radiation imprint on the CMB. The analysis brought the support of inflation to around σ = 3, which means there is a 1:1000 probability our detection of possible inflation is due to data fluctuations. Good but no cigar.

There is a way of doing general relativity pioneered by Petrov and developed further by Pirani and Penrose. This is the PPP matrix method of the Weyl tensor on the Petrov types of spacetimes. If you know general relativity but are not familiar with this it is worth learning. These various types are eigenstates of the Weyl tensor according to Killing vectors. The Killing vectors define a form of Noether conservation rule that associates symmetries with conservation laws. The types run from the D solutions that correspond to black holes, similar to a near field solution of Maxwell's equations, then type II and III out to type N that are far field types for gravitational waves. There is also a type O solution that corresponds to cosmologies. It is reasonable to think there exist instantons or excitons of these solutions or some form of quantum states. The Ernst equation bears a resemblance of this for Kerr black holes in the type D. For type N solutions these would be gravitons. There should then be the same for type O solutions --- right? The multiverse would then be a set of quantum states, either in superposition or in a mixed statistical set of states etc. The rub with this idea is that type O solutions have no Killing isometries. There are then no defined conservation laws because the Weyl tensor C_{abcd} = 0.. To get conservation laws on a cosmology you have to perturb it with gravitational waves or black holes and so forth. So there is a big open question as to how can we think of there being a multiverse with cosmologies as tunneling states from some high energy false vacuum. The best we can do is set the energy = 0, which has some correspondence with the Wheeler-DeWitt equation and work from there.

As for cosmologies bumping into each other, this is just a way they may interact with each other. This may happen as they percolate out of the false vacuum, and as quantum states they may interact with each other in certain ways. There are people looking for signatures of this in the CMB. However, we better get the B-modes first before looking for these subtle signatures.

We need to either show that a multiverse or landscape system can be constrained properly to define the world we observe, eliminate model systems on basic frame works, or if not then falsify the whole paradigm. In the last case it requires considerable work. Nothing is decided, but something like the multiverse may disagree with a person's philosophical disposition, but that is not in itself a disproof of it.

If you are a physicist, like me, you most likely do not really work in cosmology or quantum gravitation. I have published a few things on it, because if is of great interest to me. However, most physicists work in the fields of solid state physics, atomic/molecular physics and optics and laser physics or fields of this nature. I have been last working on quantum efficiencies with photons pushing electrons in solids. So is cosmology or multiverse theory “fringe?” From the perspective of physics in general it is very much a minority field. I would not though call it fringe exactly.

What general relativity tells us about black holes can't be completely right. The solutions for black holes are eternal solutions in vacua. In particular with Kerr-Newman solutions the inner horizon appears funny, and there are reasons to think there is what is called a mass inflation singularity. The term inflation has a different meaning here than in cosmology. I saw and even downloaded a paper last month or two, which for some reason I could not find in my folder, on the analysis of black hole interiors. It is extremely mathematical and over 100 pages.

The comment about Ben Carson was meant to illustrate, and Bee makes this point later, how pop-science can get twisted around. The United States is curious with a population of about 30% who think the world was created strictly according to how the first 3 chapters of Genesis in the King James Bible portray origins. For anyone familiar with Hebrew and the Torah you know there are different ways of thinking about this. Carson thinks the world emerged 6000 years ago in 6 days. I know a couple of 7th Day Adventists, which is what Carson is, and they think the Sabbath (שַׁבָּת) is the day God ceased creation and the cycle of weeks continues strictly from there. The 4th commandment then demands the Sabbath is on Saturday. It is fun to point out a bit of special relativity and how a person can fly out and return so the Christian Sunday back on Earth is on your clock the Sabbath. The 7th Day Adventist church came out of the so called Great Awakening in the 1830-40s, out of which also came the Mormons, the Baptists and the evangelical movement. It is a gift that keeps on giving. It has unfortunately snagged American society back in many ways.

The issue I have with your rationale is that it bypasses a logical scientific approach when a theory predicts fantastical, untestable phenomena, simply because the math works on what we do observe. Newtonian mechanics are still very accurate and useful for many day to day applications yet we now know the math doesn’t mimic the way nature gets to its predicted outcomes.

I think it’s more scientifically responsible to fully question things like, if GR and QM exactly mimic the way nature gets to their predicted outcomes, rather than quickly accept things like infinitely dense infinitely small massive energy, multiverses, or…the list goes on and on.

“The God hypothesis has some opposite problems; it does not arise from any existing, confirmed theories of physics, and does not explain anything (if there has to be a god to create things, who created the god--it just moves the problem back one step, leaving the why and how questions unaddressed). It shares the problem that when we look scientifically for evidence of it, none is found. ... I am for religious tolerance, but only up to the point where it conflicts with scientific evidence. ...”

You argue (correctly, I think) that deities are unscientific; so all religions “conflict with scientific evidence”. Therefore, your comments imply that we cannot tolerate any religion because all are in conflict with science (which they are.)

I must disagree; religious tolerance must be extended to all religions regardless of their merits vis-à-vis science. Only religious practices that cause actual harms (physical injury, thefts, oppression, or threats) should be denied tolerance.

Sabine, To not consider the multiverse is itself an unscientific action. There is a valid physics question - is there one unique ground state for our fundamental theory, a few possibilities or perhaps very many. If there are very many, then it is likely that a multiverse will arise in an inflationary theory. So it is clearly a scientific possibility. To discard such a physical possibility is unscientific. It also has some power of explanation about the so-called fine tuning problems of particle physics. Given your own biases, you should appreciate the idea that Naturalness of the cosmological constant and the Higgs vev are smaller problems in a multiverse. And the strong CP problem is not touched by such theories. So it provides motivation for looking for solutions to the latter problem more than the former. We do not know what directions will be fruitful. The multiverse idea motivates a class of theories with many ground states. Perhaps they will be useful to explore. Sure, these theories raise tough questions about how much we can ultimately know. But to summarily exclude them before they have been explored is an unscientific act.John Donoghue

Of course multiverse is nothing but someone’s religion… someone rendered speechless by the fine tuning of this universe. More importantly, why do all these discussions gravitate towards religion again and again? Is that because science cannot exist without religion as these irrefutable equations show:

I estimated the numbers here. I come to at most a few hundred people who work on multiverse things. That's opposed to many thousands who work on BSM and particle dark matter and inflation and so on. So I think it is correct to say that it's a small community, it's not just guesswork. You see the same thing if you look at the citation counts of papers setting the multiverse base (say, Weinberg's or Guth, Vilenkin, something like that). They're well-cited but at least a factor ten below the popular trends in BSM. Best,

"back in 2015 it was thought for a while that maybe it was clinched with BICEPII"

Had the BICEP2 results proved correct, then this would have meant the indirect detection of primordial gravitational waves, no more, no less. These are a pretty generic prediction of inflation, though strictly speaking their could be other sources. Some theories of inflation imply eternal inflation, which seems to imply the multiverse. So, it is a big leap from the indirect detection of primordial gravitational waves in the CMB to the multiverse. A big leap.

The whole BICEP2 affair was a huge PR disaster. Yes, some multiverse proponents went overboard, but that doesn't disprove the idea of the multiverse.

"I come to at most a few hundred people who work on multiverse things. That's opposed to many thousands who work on BSM and particle dark matter and inflation and so on. So I think it is correct to say that it's a small community, it's not just guesswork. You see the same thing if you look at the citation counts of papers setting the multiverse base (say, Weinberg's or Guth, Vilenkin, something like that). They're well-cited but at least a factor ten below the popular trends in BSM."

Yes, a small community, but "fringe" implies not just small but also, in this context, at least with tendencies towards being crackpot.

"For all your talk about supposed straw men, your complaint that I use Carson to show a crackpot "agrees" is a big fat straw man. I have used him to showcase what's the damage to science, not to say that what he argues is correct."

Fair enough. But Carson and people who listen to him are too far gone that any additional damage done by the multiverse would be lost in the noise.

I don't see serious problems with a multiverse in the sense as we have many galaxies, many planets or perhaps many possible vacua with broken symmetry. The laws of physics might contain this. But then we would have to go on, to specify the version relevant for what we observe, measure the relevant parameters. But that is obviously not, what is intended. The ones not accessible/observable in principle are simply irrelevant for science - like a planet, that might exist >10**11 lightyears away.I have the impression, multiverse is just trying to make a principle of what we don't know and understand: If just every possible parameter (specific detailed theory) is allowed, the theory is finished - though nothing is explained.It smells a bit as hubris: the laws and theories imaginable from what is known today have to provide the physics for all time.Seen the other way round, the (a) multiverse would be giving up to find a really scientifically meaningful predictive theory. For scientific 'practical' work it doesn't eliminate measuring the unwanted 'finetuned' parameters, the progress is purely ideological.Sometimes I think about physicists in say 1000 years. I would hope he can still achieve some progress - hard and unimaginable to us now. Who are we to decree, that they should just marvel at unaccessable multiverses helping nobody (or calculate what our current theories already allow).

Jack or John Donoghue raises a very good question. What is the physical vacuum of the universe? Is there a fundamental ground state to the multiverse or universe? We ordinarily think of the landscape as a multidimensional space with potential hills and valleys. Quantum states then evolve through barriers, inside valleys or pits, and there are then plenty of attractor regions for different configurations. These configurations are usually thought of as corresponding to Calabi-Yau compactification topologies and different ways that D-branes can wrap. In M theory there is a region with 6 points representing type I, type IIA and B, 11-dim SUGRA, E8×E8 and SO(32) connected to each other by S and T dualities or compactifications. The region in the middle, looking a bit like a stretched hide or something is M-theory and is really the unknown. We only know the boundary and the 6 string theories. We might think of there being some instantons or tunneling states across this "hide." Maybe in that picture the landscape is not so much a region with hills and valleys, but a Morse theory or Floer cohomology on a topology. In this setting there might be just one unique ground state. If not one ground state at least a much reduced number of physical vacua.

Bohm's quantum mechanics occasionally has some heuristic value. There is this pilot wave that obeys a fluid continuity equation. In Morse theory there is an g = N genus torus and we might think of the quantum state as governed by this “fluid” flowing down through this odd geometry. The fluid will find the minimum regions and then tunneling will drain out the high potential regions. This fluid or pilot wave will then make its way to the bottom. If we had something of this nature then we could say there is a unique physical vacuum or ground state to the universe. This is appealing to mathematics, but maybe if it can constrain current theory it would be of value.

Back to Ben Carson. He got his BA from Yale University and MD from University of Michigan. He clearly is not a dumb man in general. He though strikes me as a sort of savant who has a mind for one or a few things and not much else in general. His BA was in psychology, which borders on science in some ways. He took organic chemistry as I remember reading and worked hard at it. I would imagine through all of that he got some introduction to thermodynamics. He probably did not study nonequilibrium thermodynamics where one can have entropy decrease by being coupled to some other system. He makes the argument that evolution is like a tornado running through a junk yard and assembling a 747. Where do you start in arguing with such people? It also has to be pointed out that pre-biotic chemistry is a mystery and how something like ribosomes emerged is completely unknown. Run the tape of evolution backwards and you do run into a dark barrier of ignorance with how biology started.

Reply to Sean S's [... so all religions “conflict with scientific evidence”]

Lack of scientific evidence does not equal conflict with scientific evidence. As the Amazing Randi has said, "I can't prove that Santa Claus doesn't exist, I can just give a more plausible explanation for Christmas presents." God (and multiverses) could be hiding. I see a creator god as a possible concept. I just don't see any evidence for it, and in the absence of evidence, no explanatory value. (If there were evidence, e.g., actual miracles produced by prayer, the God hypothesis would explain them, at least.) I tolerated my grandmother, but I did not agree with her opinion that "if God wanted us on the moon, he would have put us there." I tolerate the hypothesis. I do not agree the way some use it or have used it to justify things such as dictatorial monarchs and slavery, on the basis that the way things are must be God's will. Tolerance does not mean agreeing with everything people do or say.

Summary: a) I disagree with the logic of your premise (that all religions conflict with scientific evidence; and in fact as a positive counter-example, see the religion known as Humanism) ; b) I do not recommend or practice intolerance or discrimination against people based on lack of current scientific evidence for their beliefs (actions that may be inspired by those beliefs are another matter); c) other than that I appreciate your comments.

Although the number of people working on the multiverse may be small, the multiverse nonsense has been popularized by Stephen Hawking, Brian Greene, Andrei Linde, Lenny Susskind, Lawrence Krauss and Sean Carroll among others.These may be a small number, but their voice is disproportionately loud and they occupy prominent positions in top universities. For whatever reason, there have been no equally prominent people loudly spreading the word that this is highly speculative nonsense as far as drawing any scientific conclusion is concerned.Hence, the damage is very great and spreading.

Maybe publishers should have a rule: Every time somebody puts a multiverse explanation in a popular article, they must compulsorily add "or maybe God made it this way. Who knows ?"I think that will convey accurately the scientific content of these ideas.

"Although the number of people working on the multiverse may be small, the multiverse nonsense has been popularized by Stephen Hawking, Brian Greene, Andrei Linde, Lenny Susskind, Lawrence Krauss and Sean Carroll among others.These may be a small number, but their voice is disproportionately loud and they occupy prominent positions in top universities. For whatever reason, there have been no equally prominent people loudly spreading the word that this is highly speculative nonsense as far as drawing any scientific conclusion is concerned.Hence, the damage is very great and spreading."

Look at it this way: their voice is disproportionately loud because they have earned their reputations (not involving the multiverse) and positions. Whom are you going to believe, Stephen Hawking or Ben Carson? Maybe there have been no equally prominent people contradicting them because all people who have got to this level actually think that the multiverse is not bunk?

"Maybe publishers should have a rule: Every time somebody puts a multiverse explanation in a popular article, they must compulsorily add "or maybe God made it this way. Who knows ?"I think that will convey accurately the scientific content of these ideas."

You are completely missing the point. Without the multiverse, there is no good answer to some claims. As a scientist, I would rather believe in the multiverse than in God.

"Perhaps the author can therefore tell us what qualifies her, as a non physicist who criticizes physicists, to adjudicate physics?"

The quote in question "Carson has misunderstood much of elementary thermodynamics and cosmology, and I have no idea why he thinks he’s even qualified to give speeches about physics." is by Sabine Hossenfelder, author of this blog, and definitely a physicist.

"This seems to be a misunderstanding on your part. What you say is technically true."

Indeed. So why do you want to further discuss it? No, Bayesian arguments don't change anything about it, they just move the issue from the distribution to the priors. I find it interesting how many people seem to think that Bayesian statistics somehow magically create information out of nothing. Point is, we only observe one universe. To the extent that you can ask what other possible things could be happening to this universe, you are under no obligation to think that these other things actually "exist".

You are flat out wrong in thinking that I don't "like" the multiverse and in any case I don't see how it matters.

"if people point you to counter-arguments..."

Like which? Look, I have addressed all "arguments" that people have raised somewhere, I just don't always repeat them all in each blogpost. That someone always repeats claims I already addressed elsewhere just tells me that there is zero learning in the system. Eg, I previously explained that just because you have a probabilistic ensemble for "what could possibly happen" doesn't mean you actually must believe all if it happens.

"This is like saying that scientists can't (yet) explain how some feature evolved, so a) evolution is bunk, Darwin was evil, hence Jesus was divine. You can't rule out a general argument on the basis of a failure of one single aspect (even if it is a failure)."

No, it is not. I am afraid you really don't understand what I am saying. It doesn't matter what else we learn about the multiverse, it won't change anything about it remaining an interpretation at best. Just try to think of any example for a possible multiverse explanation for some observable and hopefully you will see what I mean. Best,

It is certainly true that small variations in various constants of nature, parameters, etc would very probably make any sort of life impossible. This is explained well in the book by Lewis and Barnes, for example. There are several possible explanations for this. Maybe it could be no other way, but the burden of proof is on the person who claims this. Maybe it is just coincidence---could be, but theoretically we could "explain" everything by coincidence, which is not very useful. Maybe there was a creator (not necessarily some God; perhaps we are in a simulation). Maybe it evolved. And so on. To me, and apparently to at least a few well known physicists, the most likely explanations seems to be a multiverse with varying constants, so there is a weak-anthropic explanation for our existence.

Yes, we don't know the underlying distribution, but assuming some otherwise sensible distributions such that the result doesn't depend strongly on the assumption (like Weinberg's treatment of the cosmological constant which you mentioned) one can perhaps calculate some likelihood. This is probably better than essentially assuming delta functions which correspond to the values in our universe. But that is not the point. The point is that the multiverse allows for variation at all. It really doesn't matter if we are in an unlikely universe if we couldn't exist in a likely one. So probability and so on is really a red herring, or at most tangential to such arguments.

Yes, you could just claim (without evidence, of course) that there is just one universe, and things just happen to be the way they are, end of story. Hypotheses non fingo. This is a logically valid position, but not scientific in the sense that it is not useful at explaining anything. Why are inertial and gravitational mass equivalent? According to Newton, that's just the way it is. GR does offer an explanation, and most consider that more scientific. In its regime of validity, Newtoninan physics of course works fine. However, most would agree, even if it gave exactly the same results, that GR is a better theory because it explains things at a deeper level. Same here.

The Earth is at just the right distance from the Sun so that liquid water exists, which certainly makes life easier. Which is the better explanation: there is only one Earth, and it just happens to be so? Or there are many stars, many solar systems, and thus many planets, and thus no other explanation is needed other than that some will be at the right distance from the star and we shouldn't be surprised if that is where we are (even if this is an otherwise unlikely place for a planet to be).

The latter explanation is better, whether we know that other planets exist, whether some theories (solar-system formation) have them as consequences, or even in the case that someone dreamed up this idea---which could falsified in the future---with no evidence at all. Similarly, even though the multiverse is not a hypothesis, and certainly not an ad-hoc one, but rather a consequence of other theories which we have reasons to believe, it would still be a better explanation than "just so" or "God" or "coincidence" even if this were not the case.

Does the fact that Stephen Hawking and others have endorsed certain multiverse concepts equate to a religion?

Perhaps, but an alternate way to look at it is: suppose there is going to be a horse race, with many horses. Most of them you have no knowledge of, but you have studied a few of them, and of those, one seems the fastest and you decide to wager some money on it, and perhaps tell others, "If you are going to bet, this is the one I would bet on." Is that the same as having a religious belief?

We all learn language somewhat idiosyncratically. As I learned it, a religious belief means you are certain something is true. This does not seem the same to me as the above example.

The correct answer to a lot of things, including how and why our universe exists, multiverses, and which horse will win a race is, "I don't know". Nevertheless, many of us, including myself, would like to know, and will venture guesses in order to have some negligible probability of being correct.

Of course, this may also apply to some religious people, but in some religions they are not supposed to admit it. (I don't think Ben Carson would.) Also, to be fair, random occurrences may appear to be miracles and convince people like Dr. Carson. Littlewood's Law of Miracles estimates that we each experience an event whose probability is one in a million about once a month.

“Lack of scientific evidence does not equal conflict with scientific evidence.”

That’s true, but off topic. Things that merely “lack scientific evidence” are unscientific, but don’t necessarily “conflict” with science.

Religions conflict with science (and scientific evidence) because typically they either dismiss the importance of evidence (believing that knowledge destroys faith), or have entirely different ideas of what constitutes evidence (testimony, scripture, etc.) In science, “authority” is no kind of evidence, in religion is the highest form.

“Tolerance does not mean agreeing with everything people do or say.”

More than that: tolerance is almost always an attitude/behavior toward those you disagree with, or who do something you find disagreeable. If you agree, you’re not tolerating, you’re agreeing.

“... as a positive counter-example, see the religion known as Humanism ... ”

It’s not clear to me that Humanism even is a religion; it appears to be a philosophy which some adopt in lieu of a religion. Humanism has no consistent view of deities; I know of humanists who are atheists, agnostics, and Christians. I’d be surprised if there weren’t any Jewish, Muslim or Hindu humanists.

“... I do not recommend or practice intolerance or discrimination against people based on lack of current scientific evidence for their beliefs (actions that may be inspired by those beliefs are another matter) ... ”

This implies that some of Zeno's paradoxes were wrong. I read a book that attempted to prove that Zeno was wrong by going through the history of mathematics in fine detail, but it failed to convince me. I interpreted Zeno to be saying, not that an infinite sum has no limit, but that it can't be physically added up, term by term, to reach that limit since the terms never end; and that his motive was to show that what appears to us to be continuous motion is an illusion. Well, it is an illusion, if only from the fact that our visual neurons are only capable of processing a finite number of frames per second; and it might even be a fundamental illusion if time and space are quanticized.

Mathematicians have developed many clever tricks for evaluating infinite sums, but have never, to my knowledge, done so by actually, physically, adding up all of the terms. So the question I wonder about, with Zeno, is whether nature is a mathematician?

(If this were a horse race, I would be betting on your horse since you know more about metaphorical horses than I do - but without knowing why your horse is faster.)

OK, I disregarded pre-meds that majored in chemistry. I am sure they remember their basic thermodynamics quite well! Ben Carson's undergrad degree was psychology. Not to denigrate that major, nor indeed to diss the medical profession in general, but: we doctors are technicians! Techs, it is true, of the most complex machine known. Of all the aspects of this machine, the central nervous system is the most mysterious and formidable. I have great respect for Ben Carson, the neurosurgeon (just as I have respect for Mehmet Oz, the cardiovascular surgeon). I wish they would stick to their area of expertise.

"The latter explanation is better, whether we know that other planets exist, whether some theories (solar-system formation) have them as consequences, or even in the case that someone dreamed up this idea---which could falsified in the future---with no evidence at all."

Sorry, I disagree that you can reason something into existence. It smacks of ontological proofs of the existence of God. You can, of course, concoct a hypothesis, but in the absence of evidence reason alone won't tell you what is correct.

The statement I responded to was that all religions conflict with scientific evidence and therefore all religions conflict with science. That was your assumption, which I don't agree with, for reasons previously stated. Absence of science does not conflict with science. (Anti-science, which does exist, conflicts with science. As you know, there are many religious people who accept and do good science.) If you want to feel that it does, I can't stop you, but it seems illogical to me, and chiding me for not feeling the same way will not accomplish anything but annoyance.

I was introduced to Humanism in a Sunday School class, in which a video was shown of a theologian (Dr. Spong? Something like that.) discussing Humanism as one of a series of lectures about non-Christian religions. At the end, as with all the lectures, he attempted to point out the flaws vs. Christianity. My point being, you may not define Humanism as a religion, but others do, including some theologians. (At the end of the video, I thought, "That sounds great! I guess I'll be a Humanist.") (Note that Humanism includes a moral code as part of its belief system. One has to subscribe to that to be a Humanist, which I do.)

For nitpicking on personal semantics I fine myself a 10-euro donation to this site. Whether you do the same will depend on your own moral code (which might conflict with science on your terms, since there is no scientific evidence of a universal law of morality.)

“Does the fact that Stephen Hawking and others have endorsed certain multiverse concepts equate to a religion?”

No.

“... suppose there is going to be a horse race, with many horses. Most of them you have no knowledge of, but you have studied a few of them, and of those, one seems the fastest and you decide to wager some money on it, and perhaps tell others, ‘If you are going to bet, this is the one I would bet on.’ Is that the same as having a religious belief?”

No.

“We all learn language somewhat idiosyncratically. As I learned it, a religious belief means you are certain something is true.”

That meaning is too vague, too general to be valid or useful. That meaning implies your belief in what your name is would be a “religious belief”.

Using your definition, “belief” and “religious belief” mean the same thing; that’s a bad definition in exactly the same way that defining “cat” to mean the same thing as “mammal” would be a bad definition.

Beliefs are states of mind relative to something; in which you accept the truth of that something.Religious beliefs are a small subset of all beliefs, and are beliefs about deities.

Your term “certain” as in “certainly true” is extraneous.

“The correct answer to a lot of things ... is, ‘I don't know’.”

Correct.

“This implies that some of Zeno's paradoxes were wrong.”

At least some were.

“I interpreted Zeno to be saying, not that an infinite sum has no limit, but that it can't be physically added up, term by term, to reach that limit since the terms never end; ...”

What Zeno was saying is that an infinite SERIES must have an infinite sum; but we know mathematically that this is not true.

“... and that his motive was to show that what appears to us to be continuous motion is an illusion.”

I believe Zeno’s motive was to show that motion itself must be an illusion, and cannot exist; that Achilles can never catch that tortoise. I believe the idea that motion was not actually continuous was invented as a refutation of Zeno; it was supposed to cut-short the infinite series of terms. That explanation was not actually necessary at the time, but it may turn out to be true anyway.

“... if the universe will exist forever (as current observations indicate), then why are we so near the beginning?”

Well, first: there are no observations CURRENTLY that indicate our universe will exist forever, there just are no observations CURRENTLY indicating that our universe will end; so what we have is a great big MAYBE. Our universe COULD exist forever; maybe. Or eventually go ftzzz...

As for why we exist here and now, that is not so strange.

We already know that our universe has changed dramatically from its original state, it’s quite likely that we could not have come into existence much earlier.

We also know that much of what is going on in our universe right now is not infinitely sustainable, therefore our universe will change as it ages. Someday there will be that Last Star Ever. It easily could be that much farther from the beginning, it will not be possible for creatures like us to come into existence, and it might not be possible for us to survive at all.

Our existence NOW may be a small curiosity, but it does not “cry out for an explanation” unless we can show that we should have come along at a significantly different time; nothing has been found yet to indicate that.

"What Zeno was saying is that an infinite SERIES must have an infinite sum; but we know mathematically that this is not true."

To convince me of that you'll have to show me where Zeno said that. As I said, I read a mathematics book by someone who read ancient Greek and did not quote Zeno as saying that. As I read what Zeno said, it was, in the case of the Arrow Paradox, that the distance which I'll call L can be split into L/2+L/4+L/8+... (that is, the sequence L/2, L/2+L/4, L/2+L/4+L/8. etc.) for an endless series of points, and that to travel the distance L with continuous (in the Calculus sense) motion, the arrow would have to get to the end of that endless series, point by point (along with an infinite number of other points, of course). He never denied that the whole is the sum of its parts, i.e., L.

"I believe Zeno’s motive was to show that motion itself must be an illusion" (i.e., not continuous motion as I stated).

The only paradox (above) occurs if the motion is continuous. If there are discontinuous jumps, as in quantum mechanics, there is no endless series of moves - the number of moves from point to point is finite. Hence the assumption that the motion is continuous (as it appears to be, in life and when watching a movie) is implicit in the paradox, as it (the continuity assumption) was in Zeno's time.

Zeno went on to propose a solution to the paradox which was the best he could imagine at the time, which unfortunately was not quantum mechanics (and not that Achilles never catches the hare or the arrow never reaches its target; if neither of these things occurred in life there would also be no paradox) but that life itself is an illusion (/simulation) (/movie). One could fault his guess (I don't particularly, except that it is hard to falsify), but I don't fault the thinking that led to the need for a guess.

Note that Democritus also attacked the continuity assumption, but in matter rather than space.

No doubt you'll find some more semantics to nitpick in that. Language is ambiguous at best, and I'm not at the best. One can only hope that someone gets the meaning that I was trying to convey.

David Deutsch seems directly address Dr. Hossenfelder's general views on science with his comments about "going from Kepler's theories to Newton's, and from Newton's to Einstein's." Arndt keeps dodging the issue; this issue arises because we have experimental results that, as Deutsch puts it, "cannot be explained by the events that we see." Instead Arndt, the "experimentalist," seems to want to discuss pointless philosophical questions. This is exactly Deutsch's point; the Copenhagen interpretation is what he calls "bad philosophy." Indeed, that seems laid bare here.﻿ https://www.youtube.com/watch?v=kNAR74SWOho

There are some interesting things in your most recent comment, but since you labeled it as your last reply to me, it would be a waste of time to comment on them. That's too bad; I thought we were having a good chat there.

“To convince me of that you'll have to show me where Zeno said that. As I said, I read a mathematics book by someone who read ancient Greek and did not quote Zeno as saying that. As I read what Zeno said, it was, in the case of the Arrow Paradox, that the distance which I'll call L can be split into L/2+L/4+L/8+... (that is, the sequence L/2, L/2+L/4, L/2+L/4+L/8. etc.) for an endless series of points, ... ”

What you just described IS an infinite series. I could not have done much better. I don’t think any of Zeno’s works survive; we know of him only from other sources (Plato) who wrote about him.

“The only paradox ... occurs if the motion is continuous. If there are discontinuous jumps, as in quantum mechanics, there is no endless series of moves - the number of moves from point to point is finite. Hence the assumption that the motion is continuous (as it appears to be, in life and when watching a movie) is implicit in the paradox, as it (the continuity assumption) was in Zeno's time.”

In Zeno’s time, motion was believed to be continuous; hence the paradox.

“Zeno went on to propose a solution to the paradox ...”

Can you cite a source for Zeno’s proposed solution? All I can find is that Zeno’s position was based on Parmenides’ and rejected the reality of actual motion (which also excludes all change), and rejected plurality in favor of monism: that everything that exists is just one thing.

Sabine Hossenfelder says parallel universes are unscientific. I disagree. Visible object which pass an event horizon (whether that is the outer boundary of the visible universe, or the event horizon of a black hole), don't immediately cease to exist, even when we can never ever prove otherwise. That's because the mathematics (physical laws) tells you they don't cease to exist.

You can make the same claim for Schrödinger's cat. At first you see the cat alive. When you put it in a box, and the test has passed and you open the box, you see the dead of alive cat. Even followers of the collapse postulate, have to admit that until you don't open the box, the cat is in a superposition of being dead and alive at the same time. When you see the dead cat, the living cat still exists (or vice versa), but it has crossed a sort of event horizon, a region you can never reach, but which you assume to be still in existence, because the physical laws tell you this.

This should be my last response to Sean S on any issue, since he doesn't seem to understand anything I say.

Case in point: earlier I said religious beliefs are certain, to contrast them with my horse-race analogy. SS assumed that was meant to be an all-inclusive definition so that knowing your own name would be a religious belief. By my logic, this is as if I said monkeys have tails and he replied, no, you are saying cats are monkeys.

I said that Zeno did not say that infinite sums (meaning sums involving an infinite number of terms) have no limit, in the context of having a bound which they approach but never exceed. Sean S replied, "What Zeno was saying is that an infinite SERIES must have an infinite sum."

In my semantics, a,b,c,... is a series and a+b+c+... is a sum, so in one sense an infinite sum is the sum of an infinite series, but it could also mean the value of the sum is infinite, so Sean S is either stating a tautology or claiming that value of such a sum was claimed by Zeno to be infinite, which I disagree with. In the context of disputing the correctness of Zeno's Paradoxes, only the second interpretation seemed relevant, since there would be no reason to disagree with Zeno if his claim were a tautology.

I then showed how Zeno took a length L, divided it into two pieces of length L/2, then divided the second piece into two of size L/4, and so on, to show that mathematically continuous motion must go to and through an endless series of points, even though the length itself is finite. This should have explained that Zeno knew the the sum of L/2+L/4+L/8+ ... is L (not infinite), since at all steps the sum of his pieces was L, and the sum of all but the last smallest piece was only approaching that value closer and closer, not going off to infinity.

(The claim that Zeno did not understand that a sum of an infinite number of terms could have a finite limit is commonly what most people mean when they say that Zeno's Paradoxes are wrong, and I disagree with that, so I always start my defense of Zeno at that point.)

Sean S replies, "What you just described IS an infinite series." (Emphasis his.) To me this showed no understanding of the point I was trying to make.

Clearly we are not communicating, so I will stop wasting this blog's space trying. In apology for previous wasteful efforts I will make another donation to the site.

I’m sorry you feel as you do, JimV. I tried to understand what you wrote, and to respond in good faith.

I might have misunderstood some of your comments; you might not have expressed some of your points clearly. I’d provide “cases in point” but it seems you’d rather just drop it. So I’ll go along with that.

If this is how our conversation ends, I will not reply to your last criticisms beyond saying that I don’t think I’ve done anything here outside standard and accepted practices for rational discourse.

Sabine, you have talked several times about the idea that in a multiverse with universes having different constants of nature, each universe should have a probability attached to it.

I have given it some thought. It is possible that such universes are causally disconnected. There is a zero chance in this universe of a constant of nature suddenly changing. These are always fixed. There is a zero probability from our point of view that the speed of light will suddenly double at a given time at a given place, or that the Planck constant will suddenly decrease by half.

Therefore, we don't need to worry about the probabilities of other kinds of universes with different constants of nature. These might be completely separated from ours, existing somewhere "out there" (maybe in a Max Tegmark kind of mathematical universe). All we can say is that with some constants of nature, life will flourish more easily.

Sorry for coming to this late, but reading “Lost in Math” led me here just recently.

My main problem with the multiverse idea is as follows: I think the idea arises from a perceived need to understand why we seem to live in a universe finely tuned for us. I have seen the numberical values that appear “unnatural” listed in popular literature. So, to restate, I see the question as “why are the natural laws of our universe as they are?” Or, “what is the underlying nature of our universe?”

But, using the multiverse idea to get around this by saying “you have a large or infinite number of universes all with different laws of nature” so ours just exists by probability. The multiverse idea just seems to move the question of the underlying nature of things up a level. Wouldn’t the larger reality in which multiple universes come into being have natural laws that result in such a phenomenon as many universes? Is there a name in these theories for this larger reality? Forgive me if my questions are naive.

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